The transition from aluminum to copper in integrated circuit (IC) fabrication required a change in process “architecture” (to damascene and dual-damascene) as well as a whole new set of process technologies. One process step used in producing copper damascene circuits is the formation of a “seed-” or “strike-” layer, which is then used as a base layer onto which copper is electroplated (“electrofill”). The seed layer carries the electrical plating current from the edge region of the wafer (where electrical contact is made) to all trench and via structures located across the wafer surface. The seed film is typically a thin conductive copper layer or a layer of other metal. It is separated from the insulating silicon dioxide or other dielectric by a barrier layer.
As the semiconductor industry advances, future technology nodes will require extremely thin and resistive seed layers for electrochemical fill. It becomes a very challenging problem to achieve uniform electroplating thickness/current across the wafer with such resistive seeds.
When a wafer substrate is being plated, direct electrical contact to the wafer is made to the conductive seed layer in the edge region of the wafer substrate, and there is no direct contact made to the center region of the wafer substrate. The wafer acts as a cathode, at which the metal ions from the electrolyte are being reduced to form the metal layer. When very resistive seed layers are used, the potential at the edge of the seed layer at the point of electrical contact is much greater than in the center region of the seed layer further away from the electrical contact. Without compensation, this leads to significantly thicker plating at the edge of the wafer than at the center region of the wafer. The center to edge variation in potential is usually termed as the “terminal effect”. With the advance of device technology, seed layer thicknesses decrease (thus seed layers becoming more resistive), and wafer diameters sometimes increase. As a result, the difference in potential between the center and edge (due to the terminal effect) becomes more pronounced. Methods and apparatus for compensating the terminal effect, and for providing uniform center to edge plating thicknesses are needed.